Surface Engineering Of BiVO₄ For Efficient Photo-carrier Collection

Semiconductor photocatalysis technology is an effective method to alleviate the energy crisis and control environmental pollution.However,the current low energy conversion efficiency limits its application in industry.The photocatalytic conversion efficiency mainly depends on the collection efficiency of photo-carriers,that is,the ratio of photo-carriers reached the photocatalyst surface to the total amount of generated photo-carriers.Under illumination,photo-carriers generated in the photocatalyst are separated and transported driven by the built-in electric field.However,most photocatalytic materials lack a sufficiently strong built-in electric field,making it difficult for photo-carriers to reach the surface of the photocatalyst after being generated,but to recombine directly in the photocatalyst body.The surface properties of semiconductors is an important factor affecting the built-in electric field.Therefore,it is expected to improve the collection efficiency of photo-carriers by changing the surface properties of the semiconductor.In this thesis,we effectively improved the photo-carrier collection efficiency of BiVO₄-based photocatalysts by changing the surface properties and constructing heterojunctions,based on the crystal facet characteristics of BiVO₄.The specific research contents are as follows:（1）The prepared BiVO₄ film with（010）crystal facet was treated with NaOH solution.It effectively solves the problem that the BiVO₄（010）crystal facet is not conducive to the separation and aggregation of photogenerated holes.The test results show that the BiVO₄ film has a higher collection efficiency of photo-carriers after being treated with NaOH solution.This doubles the photo current density of the BiVO₄ film.The Kelvin probe atomic force microscope（KPFM）test results show that the surface photovoltage（SPV）of the（010）crystal facet is increased by nearly 6 times after the NaOH solution treatment,and it has a higher separation efficiency of photo-carriers.We deeply studied the effect mechanism of NaOH solution treatment on the separation and accumulation of photo-carrier of BiVO₄ film.The results showed that the valence band position and Fermi level of the BiVO₄film were shifted to the vacuum level after NaOH solution treatment.This will increase the band bending degree of the surface,enhance the strength of the built-in electric field in the space charge region,and provide a stronger driving force for the separation of photo-carriers.（2）A Cd S/（010）-BiVO₄ composite heterojunction structure with high photo-carrier collection efficiency was designed.BiVO₄ nanosheets with the preferential（010）facet exposed and decorated with Cd S nanoparticles were synthesized to improve the collection efficiency of photo-carriers.Our results show that the preferential（010）facet exposed BiVO₄ nanosheets have favorable photocatalytic performance for CO₂reduction compared with random facet distributed polycrystalline BiVO₄ in the same situation after combining with Cd S nanoparticles.It is attributed to the specific exposure of the（010）facet of BiVO₄ facilitating Z-schemed photo-carrier transfer in composites remarkably.An in-depth investigation of the surface photo-carrier migration by photoconductive atomic force microscopy revealed that photo-electrons of BiVO₄ tend to accumulate in the（010）facet and,therefore,facilitate the transfer to Cd S.This thesis addresses the low efficiency of photogenerated charge collection in semiconductor photocatalysis technology.The photo-carrier separation efficiency of BiVO₄-based photocatalysts is greatly improved by NaOH solution treatment and the construction of heterojunctions based on crystal facet.And the internal mechanism has been studied.